A railroad hopper car is a type of freight car having a plurality of hoppers that is used to transport loose bulk commodities such as coal, ore, phosphate, grain, and other commodities. Some railroad hopper cars are covered, and others have an open top. Covered hopper cars are used for bulk cargo that must be protected from exposure to the weather, such as grain, sugar and fertilizer. Open hopper cars are used for commodities such as coal, which can suffer exposure with less detrimental effect. Railroad hopper cars may be equipped with one or more opening doors on the underside through which the cargo is discharged. Some such cars are equipped with longitudinally oriented doors, and others are equipped with transversely oriented doors. Railroad hopper cars with transversely oriented doors are usually provided with a pair of doors for each hopper in the car, or with a pair of doors on each side of the center sill for each hopper. The development of railroad hopper cars accompanied the development of automated handling of such commodities, with automated loading and unloading facilities.
Loaded railroad hopper cars are moved into position over an unloading pit, and one or more pairs of doors are opened to discharge the contents of one or more hoppers by gravity into the unloading pit, where a conveyor is typically mounted to carry the cargo away for processing or other use. Several mechanisms are known for operating the doors of a hopper car, including the mechanisms described in my prior U.S. Pat. Nos. 5,249,531, 6,405,658, 6,955,127, 7,080,599 and 7,523,708.